In recent years, optical reproducing devices for reproducing signals such as video information and data for computers stored on recording media were required to read the data at faster speed. Further, the devices are required to have a plurality of reproduction speeds for music playback and other features.
Hereinbelow, conventional optical recording and reproducing devices will be explained.
FIG. 7 shows a block diagram of a conventional optical reproduction device [such as disclosed in U.S. Pat. No. 6,028,826. ] A light beam emitted from a light source 101 such as a semiconductor laser is formed into a collimated light beam through a collimator lens 102. Thereafter, the collimated light beam is reflected through a polarizing beam splitter 103, passes through a quarter-wavelength plate 104 and is focused through a convergent lens 105 on a disc 107 rotated by a motor 111. The light beam reflected from the disc 107 passes through the convergent lens 105, quarter-wavelength plate 104, polarizing beam splitter 103, condensing lens 108 and then is irradiated on a light detector 109. The convergent lens 105 is mounted on a movable portion of an actuator 106. The convergent lens 105 is adapted to shift perpendicularly to the disc surface by the flow of current through the focusing coil of the actuator 106 and to shift in the radial direction of the disc by the flow of current through the tracking coil of the actuator 106. In a head unit 110, there are mounted the convergent lens 105, actuator 106, quarter-wavelength plate 104, polarizing beam splitter 103, collimator lens 102, light source 101, condensing lens 108 and light detector 109.
The output of the light detector 109 passes through amplifiers 114a-114d and then is input to a focus error circuit 115. The focus error circuit 115 processes the output signals from the amplifiers 114a-114d and outputs a focus error signal corresponding to the vertical displacement of the focal point of the light beam from the information-recording surface of the disc. The focus error signal is applied to the focusing coil of the actuator 106 via a variable amplifier 116, a phase compensator 117 for phase compensation, a switch 118 for the on-off control of focus control by the controller 150 and a driving circuit 119 for the amplification of power. Thereby, a control is performed such that the focal point of the light beam may be positioned on the information-recording surface.
Also, the output signal of the light detector 109 inputs to a tracking error circuit 120 through amplifiers 114a-114d. The tracking error circuit 120 outputs a tracking error signal corresponding to the positional displacement of the focal point of the light beam from a track. The tracking error signal is applied to the tracking coil of the actuator 106 through a variable amplifier 121, a phase compensator 122 for phase compensation, a switch 123 for the on-off control of the tracking control by the controller 150, and a driving circuit 124 for the amplification of power. Thereby, the convergent lens 105 is controlled such that the focal point of the light beam may be positioned on the track.
Further, the output signal of the light detector 109 is input via an adder 126 to an equalizer circuit 130, which functions as an amplifier and a frequency band limiter in identified frequency bands. The equalizer circuit 130 reproduces a RF (Radio Frequency) signal. The reproduced RF signals are input to a PLL (Phase Locked Loop) circuit 131, which produces a clock used as a reference in the reproduction process for signals. The PLL circuit 131 provides a reference clock synchronized with the RF signals.
FIG. 2 shows a block diagram of the PLL circuit 131.
A binarization circuit 301 binarizes the RF signal input to the PLL circuit 131 and outputs an RF binary signal DT. Also, a phase comparator 302 compares the phase of the RF binary signal with that of a lead clock and outputs a signal corresponding to the phase difference obtained. The output signal is amplified by an amplifier 303 and corrected for phase by a phase corrector 304 and input to a variable oscillator 305. The variable oscillator 305 makes its oscillation frequency vary according to an input signal and outputs an oscillation signal. The oscillation signal is input to a frequency divider 306. The frequency divider 306 divides the frequency of the oscillation signal according to a specified divide ratio and outputs the divided signal as a lead clock CK.
Further, the output signal of the light detector 109 is applied to a reflected-light-quantity detector 127, which outputs a signal responsive to the light quantity of the reflected light incident on the light detector 109.
A driving circuit 112 is intended to drive a disc motor 111 based on the output of the controller 150. The controller 150 controls the number of rotations of the disc motor 111 to be a predetermined number.
For example, in a device configured for reproducing compact discs (CD) at multiple reproduction speeds such as a standard, two-times, and six-times speeds, the settings of the focus control, tracking control, optimum gain of the PLL circuit, phase compensation characteristic and divide ratio are different at each reproduction speed, respectively.
For the reasons described above, conventional devices have performed the signal processing as follows.
When a reproduction speed is changed, the controller 150 sends to the driving circuit 112 a command for setting the number of rotations of the motor 111 to be a predetermined number. Also, the controller 150 provides signals to the variable amplifiers 116 and 121, PLL circuit 131 and equalizer circuit 130 for setting them to the characteristics (i.e. frequency or filter) corresponding to the reproduction speed.
Further, when the focus control is lost due to surface flaws on the disc and vibrations of the device, the abnormal driving sometimes damages the actuator 106. For this problem, conventional systems have used exclusively the reflected-light quantity detector 127 for detecting an improper operation out of focus control and switched off the focus control.
The conventional devices described above have the following problems. At first, in the reproduction method using CAV in which the motor rotates at a constant rate at all times, the (frequency) characteristic of the signal processing system is not varied depending on a position in the radial direction of the disc. Since the reproduction speed is different depending on the radial position during CAV reproduction, the optimum characteristic of the signal processing system also becomes different on the radial position. Therefore, there has been a problem that the quality of reproduced signals is sometimes too wrong to make reproducing operation impossible in some radial position.
Secondly, in the CAV reproduction method in which the number of rotations of the motor 111 is controlled depending on the reproduction position of the disc so as to maintain the reproduction speed constant, the frequency characteristic of the signal processing system is not varied during reproducing. When the heat unit 110 is quickly sent from an inner circumference to an outer circumference or vice versa to restart reproduction, the rotational change of the motor 111 sometimes cannot follow such a quick change. As a result, the reproduction is performed at a different speed from the predetermined reproduction speed. Therefore, there has been a problem that the difference from the predetermined reproduction speed is sometimes too large to make reproduction operation impossible.
Thirdly, in the method in which reproduction speed is varied, all characteristics of the signal processing, focus control and tracking control systems have been varied at the same time on setting a target rotation of the motor. However, the motor cannot actually respond quickly to such change, and there has been a problem that, during the transition, the reproduction speed is so different from an optimum characteristic that reproduction is impossible.
Fourthly, because the operation out of focus control has been detected exclusively by the reflected-light quantity detector, the characteristics of AS (an output level of reflected-light-quantity detector 127) varies broadly with respect to the focal point due to stray light and so on. And further, for discs with low reflectivity such as CD-RW, the S/N ratio is so small that problems of improper detection and non-detection have existed.
For overcoming the problems described above, the present invention is directed to provide a stable control and a stable reproduction performance, by varying the characteristic of the control system to the suitable characteristic responsive to the reproduction speed. Further, the present invention is directed to select the signal for detecting an operation out of focus control from three kinds of signals responding to the conditions of the disc to be reproduced and the device, and to thereby realize a stable and high speed detection of an operation out of focus control. Therefore, the collision of a convergent lens to discs causing flaws on the discs and fractures of actuator can be prevented.